1. The Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a non-aqueous secondary battery using an electroconductive or semiconductive polymeric material as a positive electrode active material.
2. Discussion of Background
It is conventionally known that polymeric materials such as polypyrrole, polythiophene, polyphenylene and polyaniline, which are insulating or semiconductive materials themselves, become electroconductive just like metals by doping impurities therein, as disclosed in A.F. Dinz. J. Chem. Soc., Chem. Commun., 1975. 635; Japanese Laid-Open Patent Application 56-47421; Electrochem., Acta., 27, 61 (1982); and F. Diaz. J. Electroanal. Chem. 111. 1524 (1980). The above-mentioned doping can be carried out reversibly, accompanied with color change, so that research and development activities are now being directed to the application of the above polymeric materials, for example, in the field of a display device, a secondary battery, an electromagnetic shielding material and a variety of sensors.
In line with the trend toward small-size, light-weight electronic devices, the thinning and lightening of the batteries used as power sources for these devices has become an important research subject. Therefore, the application of the above polymeric materials to a light, small and thin secondary battery is particularly expected. Because the secondary batteries using the above polymeric materials as electrode active materials show high energy density and are more flexible than conventional batteries employing conventional active materials. The aforementioned polymeric materials can be used as positive electrode active materials of non-aqueous secondary batteries, because doping and dedoping can be carried out in a non-aqueous solution.
In addition, the need for a battery with large energy capacity has become evident, and alkaline batteries from which high voltage can be gained have been developed in recent years.
A secondary battery comprising an alkali metal such as Li and Na serving as a negative electrode active material, and the above-mentioned polymeric material serving as a positive electrode active material is thus expected to have high energy density.
However, the above-mentioned alkali metal battery employing an electroconductive polymeric material as a positive electrode active material is put into practice only as a coin-type battery. With such circumstances taken into consideration, it is said that the advantages of the above-mentioned polymeric materials are not effectively utilized in the secondary battery at present. In other words, a light, flexible sheet-shaped secondary battery having an increased surface area has not been realized yet.
In the battery using the electroconductive polymeric material as an active material for the positive electrode, smooth doping and dedoping is closely connected with the improvement in the characteristics of the battery. The doping and dedoping action is considered to be determined by the interaction between the employed electrolytic solution and polymeric active material. In other words, the electrolytic solution significantly affects the performance of the polymeric active material.
In the light of the above-mentioned interaction between the electrolytic solution and polymeric active material, an electrolytic solution capable of smoothly conducting the doping-dedoping reaction of the electroconductive polymeric active material is required in order to obtain a secondary battery having a large energy capacity and a high electric current density during the charging and discharging operation.
On the other hand, since the compatibility of the electrolytic solution with the negative electrode material is also important, the electrolytic solution which does not have adverse influences upon the negative electrode must be selected.
As for the materials of the negative electrode, alkali metals including lithium in particular have been actively studied in order to obtain batteries having high energy density. However, when lithium is used as the negative electrode active material of the secondary battery, a reaction product of lithium and an electrolytic solution, which has not yet clarified, is unfavorably deposited in the form of dendrite or moss on the negative electrode, or lithium is finely divided in the course of doping and dedoping action. This will shorten the cycle life of a battery. It is confirmed that the frequency of the occurrence of the above phenomena varies depending on the types of the electrolytic solution. Therefore it is necessary to choose a proper electrolytic solution for the negative electrode material, which has no potentialities to induce the above phenomena, in order to obtain the secondary battery having a prolonged cycle life.
Conventionally, propylene carbonate (PC) is mainly used as a prime solvent of an electrolytic solution of the secondary battery. This is because the propylene carbonate based electrolytic solution can actually attain high stability and reliability when used in a primary battery. However, the solubility of an electrolyte in the propylene carbonate based electrolytic solution is poor, so that the highly-concentrated electrolytic solution cannot be obtained. In addition to the above, the solution viscosity of the propylene carbonate based electrolytic solution is so high that it is difficult to cause this electrolytic solution to penetrate into a separator when a second battery including a separator is prepared.
As is apparent from the above, the electrolytic solution of the secondary battery which matches very well with both the positive electrode active material and negative electrode active material is a key point to prepare a secondary battery having a large energy capacity, high electric current density in the charging and discharging operation and has a prolonged cycle life.